The invention relates to a system and method for mobile communications, especially for dynamic radio channel allocation in a mobile communications network
In a digital mobile network the ability to property receive and decode a radio signal depends on the carrier-to-interference strength ratio C/I at the receiver. Clearly, a too low C/I will result in bad quality or the total loss of the radio connection. On the other hand, radio communications quality does not become significantly better for a very high C/I ratio since the transmission method is designed to cope with a certain amount of noise such that above a certain C/I level a received signal can be properly demodulated and decoded. However, a too high C/I does not maximise capacity of the network. Either the carrier strength C should be lowered to reduce the interference generated to other receivers or more interference should be permitted to be generated by other transmitters. This provides a means to get more capacity out of the available radio spectrum, Correspondingly, excessive C/I translates to a loss in capacity.
The application WO 97/32444 provides a method for allocating frequency channels to cells in a cellular telephone system. The application relates to automatic allocation of frequency channels to cells in a cellular telephone system. The uplink signal quality is measured in terms of uplink interference level. For selected frequency channels, uplink interference levels are measured by the corresponding transceivers. The signal to interference ratio for the radio station is calculated from the measurements of the uplink signal quality and the uplink interference level.
Another application U.S. Pat. No. 5,594,949 provides a method and apparatus for locally estimating the interference on downlink channels available to a base station to determine candidate channels for new calls. When a new connection is to be established with a mobile station, the base station signals those mobile stations already connected to make interference measurements. These measurements are then used to estimete the interference which a new connection will have in the downlink. The base station measures the received signal strength of a mobile station on the control channel and based on that measurement, estimates the signal strength which the mobile station wit receive from the base. Based on said interference measurements and said signal strength measurement the base station BTS then calculates the carrier-to interference ratio.
This leads to the well-known ultimate goal that C/I should be homogeneously distributed over all receivers in the network at any point in time.
However, in current GSM networks (Global System for Mobile communication), this goal is far from being realised. The following statements summarise the current status quo:
The frequency plan is fixed, i.e. each base station transceiver (TRX is being assigned one frequency or one frequency hopping sequence. This prevents the allocation of a channel, i.e. frequency and TDMA (time division multiple access) time slot (TS), to a mobile station (MS) according to the criterion of spreading out C/I. In general, handover (HO) and power control (PC) decisions are not based on C/I, but on other less adequate quantities such as field strength (FS) and quality (meaning bit error rate). Some C/I measurements can be provided by base stations BTS, but they are limited and for uplink direction (MS to BTS) only. For neighbour cells, only FS measurements on the BCCH frequency (control channel) are performed. HOs are made without direct knowledge of the radio conditions on non-BCCH frequencies.
Frequency hopping (FH) provides statistical interference spreading in time, but no active interference management is currently implemented,
In the soft-capacity enhancement feature Intelligent Underlay Overlay (IUO), the evaluation of C/I is done on a cell-by-cell basis and averaged over all 8 TSs of a TDMA frame. Here, C/I represents a worst case scenario rather than the actual C/I at a MS. In the conventional concept of Automatic Frequency Planning (AFP), the fixed frequency plan is periodically improved according to C/I criteria. C/I is calculated from the live network traffic, but, as with IUO, the resulting C/I matrix refers to interference between cell areas, not as experienced by the MSs themselves. Additionally, there remains the major problem to get the huge amount of measurement data from the base station controller BSC to an external AFP tool. In conclusion, in GSM networks today C/I is not homogeneously distributed over the receivers.
The proposed solution of the invention improves upon current networks, completely with the domain of GSM. The major benefits are:
C/I is determined at each MS and is continuously tracked. This allows the network to detect insufficient or excessive C/I for each MS and furthermore to assess the overall downlink C/I distribution of the network. Local and global interference management is made possible.
Handover, i.e. HOs and downlink Power Control (PC) are based on C/I criteria. The network compares the effects that potential HOs or downlink PC decisions would have on all the MSs which would be affected by such a decision. Thus, HOs and downlink PC decisions are C/I-based. There is less risk for dropped calls due to interference. Due to such C/I-based HOs, the network can increase C/I for MSs with too-low C/I and decrease C/I for MSs with too-high C/I, thus homogenise C/I across all MSs, in order to come as close as possible to the most homogeneous C/I distribution.
There is basically no frequency planning except for the BCCH Frequencies are allocated, as required, for channel allocation and HO as determined by C/I consideration. Each TS within a TRX can be allocated a different frequency as opposed to having fixed frequency assignments per TRX. There is no frequency hopping (FH), i.e. the frequency used on a channel does not generally change from frame to frame.
The neighbour cell list each MS receives after each HO is performed is specified as follows. MSs can be given dedicated neighbour lists according to various criteria, such as C/I, speed, traffic, rapid field drop, etc. This makes possible the management of different overlaying network layers, e.g. macro and micro layers, or Where reporting of different sets of neighbour cells is required to make an optimal channel allocation decision. Downlink C/I spreading is only constrained if local traffic exceeds the local hard capacity limit, given for each cell by the number of installed TRXs.
All this C/I spreading results in substantial capacity and quality gains. An effective frequency reuse between 3 and 3.5 without sacrificing quality is expected. Capacity and quality are balanced according to the actual traffic.
Interference management is focused on the radio conditions at the MS itself instead of according to a cell average. In this regard the network can be viewed as a xe2x80x9csingle logical cellxe2x80x9d, with the MS being monitored along its own trajectory through the network.
The invention can also be considered as a vastly improved IUO without planning, where C/I measurements now represent the actual behaviour at a MS.
According to a first aspect of the invention there is provided a method for a radio channel allocation in a telecommunication networks, comprising fixed transceivers and mobile radio stations, the method being characterized in that it comprises the steps of
calculating a signal to interference estimate for the radio station, and
performing channel allocation for a particular radio connection to a radio station based upon the calculated signal to interference estimate for the radio station.
According to a second aspect of the invention there is provided a mobile communications network comprising
a number of base stations, each base station being capable of transmitting radio signals to and receiving radio signals from the area of an associated cell for communication with
a mobile station in the associated cell,
a base station controller to which a number of said base stations are connected, the network being characterized in that the it comprises
means for calculating a signal to interference estimate for the mobile station, and
means for performing channel allocation for a particular radio connection to the mobile station based upon the calculated signal to interference estimate for the mobile station.
According to a third aspect of the invention there is provided a base station controller for supervising a number of base stations connected to the base station controller and for supervising communication to mobile stations being connected to one of the base stations over a radio connection, the base station controller being characterised in that the it comprises
means for calculating a signal to interference estimate for the mobile station, and
means for performing channel allocation for the radio connection to the mobile station based upon the calculated signal to interference estimate for the mobile station.
At the network the C/I ratios of the different mobile stations MS are determined by the base station controller BSC. Already in present networks, most of the data required to calculate the downlink carrier strength C and the interference I exists in the BSC. The BSC knows which BTS transmits at which frequency and at what transmission power.
Relating this information to each MS""s field strength measurements on the serving channel and on the BCCH frequency is enough to calculate the actual and potential C/I ratio.
A requirement to know the relative timing between the transmissions of different BTSs is not available in standard GSM systems where BTSs operate autonomously and where their transmissions are not synchronised with each other. The invention can be used to align the time slots of different BTSs connected to one and the same BSC. Actually, the invention provides the justification to implement Time Slot Alignment.
Further benefits of the invention are given for achieving a self-regulating network:
The need for frequency planning mostly vanishes. AFP becomes unnecessary, since non-BCCH frequencies are not pre-assigned, but allocated in a truly dynamic way. The remaining BCCH frequency planning can much more easily be automated with the invention in place and probably will not require conventional AFP.
The invention supersedes IUO. Thus, the fairly complicated and time consuming planning effort for IUO is not needed.
The invention works without FH. Thus, all planning related to FH or IFH (=intelligent frequency hopping), e.g. the choice of hopping sequences and hopping sequence numbers, is not needed.
HOs and downlink PC are much simplified. There are fewer parameters, and most of them can be planned and refined more easily, since they relate more closely to interference control, network quality, traffic control and hard capacity. Conventional HOs and PC methods may still be required to deal with inter-BSC HOs and special situations such as uplink problems, but since they will be invoked much more seldom, the need for refining parameters is reduced.
Traffic control, i.e. dynamic shifts of capacity between cells by means of traffic handovers become much easier automatable, since there is direct relationship between soft capacity and the C/I target, which controls the HO and PC processes. It is possible to classify subscribers with different rates according to the different carrier to interference ratio.
Considering this all together, time slot alignment together with the allocation according to the invention provide a major step towards a Self-Regulating Network, where most network parameters are either not necessary any more or are automatically tuned by the network according to measurements and statistics gathered in the live network.